The present invention relates to a suspension control system having a damping characteristic inverting type shock absorber for use in vehicles such as automobiles and railway vehicles.
One example of conventional suspension control systems having a damping characteristic inverting type shock absorber is disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 2002-321513.
The suspension control system disclosed in the above-mentioned publication operates to damp vibrations of the sprung mass (vehicle body) as follows. When the vehicle rides over an upward swelling of undulation in the road surface, for example, the suspension control system provides “soft” damping characteristics for the compression stroke, thereby suppressing the upward movement of the vehicle body as it is pushed up. Meanwhile, “hard” damping characteristics are provided for the extension stroke. Therefore, when the vehicle goes down the undulation after reaching the top of the swelling, the vehicle body that tends to move upward is pulled downward by the weight of the axle-side members of the vehicle, thereby keeping the vehicle body flat. In addition, the conventional suspension control system estimates the piston speed (relative velocity) from the vertical acceleration of the sprung mass and suppresses unsprung mass vibrations (violent vibrations of the unsprung mass) on the basis of the estimated relative velocity, thereby minimizing high-frequency noise to the occupants of the vehicle and the degradation of tire's road holding capability (and hence the degradation of steering stability during cornering), which are caused by unsprung mass vibrations (violent vibrations of the unsprung mass).
Incidentally, the above-described unsprung mass vibrations (violent vibrations of the unsprung mass) have a frequency (unsprung mass resonance frequency) in the neighborhood of about 10 to 15 Hz and are, in general, higher in frequency than vibrations occurring on the sprung mass (sprung mass resonance frequency; about 1 Hz). The above-described prior art attempts to suppress the unsprung mass vibrations but performs the control for suppressing the unsprung mass vibrations on the basis of the vertical acceleration of the sprung mass. Therefore, the prior art suffers a delay in change of the damping characteristics of the shock absorber and, in actuality, cannot perform unsprung mass vibration damping control with high accuracy. Thus, there is some room for further improvement in the suppression of unsprung mass vibrations.
The present invention was made in view of the above-described circumstances. Accordingly, an object of the present invention is to provide a suspension control system capable of improving the effectiveness of the unsprung mass vibration damping control.